Most electronic circuits are implemented within microelectronic circuits, commonly referred to as “chips”. Such a chip comprises a semiconductor die carrying the microelectronic circuit encapsulated within a plastics case. This enables the chip to be bonded or soldered to circuit boards and the like for the formation into more complex products. Many applications of microelectronic circuitry may require interfacing from a relatively low voltage side, where for example the supply rails may differ from each other by only a few volts, to higher voltage components as might be found in the energy, signaling, automation, communications or motor control arenas. There are also safety critical applications such as medical applications, where high voltages must not be allowed to propagate from the circuit towards a patient being monitored. Although these high voltages may not be generated deliberately, they might occur in certain fault modes, for example if a power supply were to develop a fault. It is known to isolate low voltage and high voltage sides of a circuit from one another using “isolators”. These have typically involved discrete components, such a signal transformers, being mounted on a circuit board between a low voltage side of the board and the high voltage side of the board. More recently “chip scale” isolators have become available. Within a “chip scale” isolator the low voltage and high voltage sides of the circuit are provided within a plastics package of the type known in the provision of integrated circuits, such as a dual in line package.
The reduced dimensions in chip scale isolators start to give rise to breakdown mechanisms not seen in non-chip scale, i.e. discrete component isolators. Isolators are often given a rating called the breakdown voltage. When the voltage across electrodes of the isolator exceeds the breakdown voltage, the dielectric material between the electrodes may exhibit electrical breakdown and become electrically conductive, no longer performing as an effective isolator.
To increase the breakdown voltage, an isolator can be manufactured with a thicker layer of dielectric material between the electrodes.